1. Field of the Invention
The present invention relates to a liquid-crystal display suitable for a wide viewing angle, particularly to a liquid-crystal display having a liquid-crystal layer oriented to bend alignment and suitable for an optically compensated birefringence (OCB) mode.
2. Description of the Prior Art
A liquid-crystal display is rapidly spread as a display unit substituted for a CRT because it is thin and a display area can be relatively easily increased.
A liquid-crystal operation mode includes a twisted nematic mode (hereafter referred to as TN mode). The TN mode is realized by rotating the direction of the molecular axis of liquid-crystal molecules (hereafter referred to as director) by approx. 90° between substrates and twist-orienting the liquid-crystal molecules. When applying an electric field vertically to a substrate, a director vertically rotates to display an object.
However, the TN mode has a problem that a viewing angle is narrow. Therefore, it is impossible to visually confirm a displayed object from a diagonal direction. Moreover, when a screen area is increased, an object is not properly displayed because appearance of the object differs at the center and an end of a screen when viewing the screen from a certain viewpoint in a diagonal direction.
To solve the above problem, a viewing angle is expanded by adding a phase compensation plate to a TN-mode liquid-crystal panel in the case of the official gazette of Japanese Patent Laid-Open No. 75116/1994. In the case of this art, however, it is difficult to completely compensate a twisted structure intrinsic to the TN mode and therefore, the problem is not fundamentally solved yet.
Therefore, OCB (optically compensated birefringence) is noticed as means for improving a viewing angle.
OCB is realized by forming a liquid-crystal layer oriented to bend an alignment between two substrates and moreover, setting a phase compensation plate for compensating a phase of the liquid-crystal layer outside of each substrate as shown in FIG. 1 to be mentioned later.
A liquid-crystal layer oriented to a bend alignment represents that liquid-crystal molecules held between two substrates show a symmetric orientation from the center between the substrates as shown in FIG. 4C to be mentioned later. Moreover, directors of liquid-crystal molecules are changed by applying a voltage between the substrates.
Moreover, a phase compensation plate having a negative birefringent property is known which is disclosed in the official gazette of Japanese Patent Laid-Open No. 294962/1994. Furthermore, a biaxial phase compensation plate is reported by Kuo et al. in an article titled “Improvement of Gray-Scale Performance of Optically Compensated Birefringence (OCB) Display Mode for AMLCDs” on pp. 927 to 930 of SID'94Digest issued on Jun. 14, 1994. Furthermore, a hybrid-arranged phase compensation plate having a negative birefringent property is known which is disclosed in the official gazette of Japanese Patent Laid-Open No. 197862/1998.
In the case of OCB, when changing directors of liquid-crystal molecules by applying a certain voltage, two types of retardations such as retardation Rlc and phase-compensation-plate retardation Rrf are obtained. When retardation R of the whole OCB obtained by integrating these two types of retardations Rlc and Rrf is equal to zero or a multiple of a wavelength, black is displayed. In the case of a voltage other than the above, white or halftone is displayed.
A liquid-crystal layer oriented to the bend alignment does not include any twist differently from the TN mode. Therefore, phase compensation is easily made and wide-field display is realized.
However, the above conventional OCB has the following problem.
That is, in the official gazette of Japanese Patent Laid-Open No. 197862/1998, the product between a birefringent index Δn of a liquid-crystal material in a cell and a thickness d of the cell is set to a value between 790 nm and 1190 nm. This value is a value when every liquid-crystal molecule is parallel with a substrate.
When a bend-alignment state is realized, a liquid-crystal molecule at the central portion rises. Therefore, the retardation Rlc of a liquid-crystal layer decreases to about ⅓ to ½ of the above value (790 to 1190 nm).
The value of the retardation Rrf of a phase compensation plate is not specified. However, when considering that black display is obtained at a high voltage of approx. 8 V and referring to the value of retardation of a currently-marketed hybrid-aligned negative birefringent phase compensation plate, the retardation Rrf is equal to approx. 100 nm.
In this case, the major axis of the birefringent index of the phase compensation plate is orthogonal to the major axis of the birefringent index of a liquid-crystal molecule. Therefore, the retardation R of the whole OCB when displaying white becomes approx. 250 to 300 nm. A transmitted-light intensity I of a liquid-crystal display using the birefringent property can be expressed by the following equation (1),I=A·(sin(2·θ))2·(sin(R·π/λ))2  (1)where A denotes a proportionality factor, θ denotes an angle formed between polarization axis and birefringent-index major axis of a polarizing plate, and λ denotes a wavelength of light. From equation (1), it is found that light having λ of 500 to 600 nm has a high transmittance when setting the retardation R to 250 to 300 nm. That is, setting is made so that light having a green wavelength band is well transmitted.
Since a human eye has a high visibility in green wavelength band, brightness rises in the case of the conventional OCB disclosed in the official gazette of Japanese Patent Laid-Open No. 197862/1998.
In the case of the above OCB, however, the following trouble occurs particularly when performing color display.
Transmittances of red, green, and blue lights when using OCB are shown in FIG. 10 to be mentioned later. That is, transmittances of green and red lights monotonously decrease as an applied voltage rises. However, transmittance of blue light first increases, peaks at 2.6 V, and thereafter decreases. Therefore, to display gradations, a voltage of 2 to 10 V is applied to red and green lights. However, in the case of blue light, an applied voltage of 2.6 to 10 V must be set differently from the case of green and red lights.
In the case of a general liquid-crystal display, when applying a voltage to liquid crystal, it is preferable to apply the same voltage to red, green, and blue. This is because, if a different applied voltage is set to each color, the number of electronic components increases to obtain a desired voltage.
Therefore, to set a proper voltage, the number of electronic component increases, the manufacturing cost increases, and moreover a circuit substrate increases in size, and thus it is prevented to make a compact liquid-crystal display device.